We have demonstrated that estradiol receptor from two lines of rat mammary tumor MTW9 differ by chromatographic, kinetic and DNA binding properties. MTW9-MtT does not regress after ovariectomy, MTW9-P does regress. MTW9-MtT, the rat mammary tumor grown by co-implantation of the mammosomatotropic tumor MtTW10 shows ovariectomy-induced regression when MtT is resected. MTW9-MtT tumor cytosol receptor shows much less binding to DNA than does receptor from MTW9-P; after resection of MtT, DNA binding is similar. These data suggest that the differences between tumor receptors observed in our animal model may allow differentiation between tumors which have estradiol receptor and either regress or fail to regress after ovariectomy. We propose to study human breast cancers which contain estradiol receptor by the techniques described which differentiate responsive from non-responsive tumors. Differences in receptor behavior would then be correlated with response to endocrine treatment. If such study of estradiol receptors in mammary cancer would identify hormone-responsive tumors, current therapy would be improved.